With development of advanced synthetic chemical technologies, polymer materials are broadly used in medical products including textiles and other various commonly used products such as a flame-retardant agent, etc. However, among polymer materials commercially available in the art, in particular, polyolefin resin, halogenated resin and polyimide resin are unstable with respect to heat or light and, in a case where they are molten through heating or used at a high temperature during a molding process, thermal degradation or quality deterioration may be caused. Due to this, a final product may encounter disadvantages such as coloring problems, modification of characteristics, deterioration in mechanical properties, or the like.
Accordingly, in order to endow heat resistant stability to the resin formed product, heavy metal fatty acids such as Pb, Cd, etc., Sn materials and/or Ba materials have been used alone or in combination with two or more thereof. However, since the toxicity of such a stabilizer has recently become a social and/or environmental problem, Sn-based and Ca—Zn based stabilizing agents are being intensively developed. However, such stabilizing agents also entail problems, for example, toxicity problems. In order to improve the above problems, some have proposed techniques using a composite metal hydroxide such as Mg—Al hydrotalcite, Li—Al hydrotalcite, etc.
Hydrotalcite is a natural mineral discovered in 1842 and was reproduced by synthesis in a laboratory by Feiknecht in 1930 which is 100 years after a general structure thereof was disclosed. A layered composite metal hydroxide, known as a layered double hydroxide (LDH) or layered mixture metal hydroxide, including an anionic layer, is a two-dimensional layered structure enabling a charge balance by using anions, which are exchangeable with a positively charged ionic layer including two kinds of cations, wherein the anionic layer and can be exchanged with a variety of organic/inorganic ions.
Such a layered composite metal hydroxide is commercially synthesized with a high purity and applied to a wide range of applications, for example, an environmental pollution remover, a catalyst inactivating agent, an acid scavenger, an acid adsorbent, a flame retardant, a flame-retardant assistant, a heat-resistant stability enhancer for polymers, an acid-neutralizing agent, a UV ray protector, a heat preservation agent, and so forth.
Preparation of the layered composite metal hydroxide may be generally classified into co-precipitation using an aqueous metal salt and hydrothermal synthesis using a poor water-soluble metal hydrate to conduct synthesis at a high temperature.
U.S. Pat. No. 3,879,523 discloses a method for synthesis of a layered composite metal hydroxide having a hydrotalcite structure by using a variety of mixed metal components and anion providing materials, which is a process to synthesize the above material through co-precipitation using a water soluble metal salt, thus requiring excess washing water in a reaction process and causing a problem of generating by-products in large quantities.
Meanwhile, U.S. Pat. No. 4,458,026 discloses a method for synthesis of a hydrotalcite type catalyst, which includes: adding a solution containing a bivalent metal inorganic salt such as magnesium nitrate and a trivalent metal inorganic salt such as aluminum nitrate mixed together to another solution containing sodium hydroxide and sodium carbonate, as an anion providing material, combined in a stoichiometric ratio thereof; reacting the mixture to synthesize a hydrotalcite slurry; thereafter, filtering and washing the slurry, and heating and drying the same at a temperature of about 300 to 600° C. The method described in the above US patent has a problem of consuming a considerable amount of heat during processing.
Alternatively, U.S. Pat. No. 4,904,457 discloses a method for synthesis of hydrotalcite at a high yield of 75% or more, which includes: heating a magnesium compound such as magnesium carbonate or magnesium hydroxide at a high temperature to activate the same; and reacting the activated magnesium in a solution containing hydroxyl group ions as well as an aluminum salt and a carbonate salt in a pH range of more than 13. However, the process proposed in the above US Patent not only consumes excess energy but also requires a reaction under an alkaline compound condition, thus causing a significant decrease in a thermal stability efficiency of the finally produced hydrotalcite.
Meanwhile, Korean Patent Laid-Open No. 2001-0108920 discloses a method for synthesis of a layered composite metal hydroxide in a hydrotalcite form, which includes: dispersing a crystalline metal hydroxide mixture in water; mixing the dispersion with a water-soluble metal hydroxide mixture, an interlayer anion providing material and a solution containing alkali-metal hydroxide; thereafter, synthesizing a hydrotalcite type layered composite hydroxide under high temperature/high pressure conditions. Although the above Korean published Patent describes that using advantages/disadvantage of both co-precipitation and hydrothermal synthesis can control physical/chemical properties of the layered composite metal hydroxide after final production thereof, a water-soluble metal hydroxide in excess is used and causes by-products in large quantities and a reaction at a high pH, thus causing disadvantages such as water pollution and requirement of excessive washing.
Alternatively, Korean Patent Laid-Open No. 2000-0049194 discloses a method for synthesis of a composite metal hydroxide containing lithium metal as a stabilizer for a halogen-containing resin. However, since three different metals as well as lithium as an expensive metal are used, economical effects are decreased.
In addition, when a Mg—Al laminated structure includes other impurities, crystallinity is generally reduced to cause a deterioration in performance (see Microporous and Mesoporous Materials 111 (2008) pp. 12-17).